Caoimhinn is the son of former Derry City defender, Liam, and played with Derry's title winning A Championship side of 2011. For the first team, he made the bench one occasion in 2011, against Bohemians on 28 October. Caoimhinn's league debut came on the 10th September 2012 as a late substitute away to Drogheda United, his only league appearance of that year.

Caoimhin then signed for Scottish side Kilmarnock but after one year there he returned to Ireland in January 2014 and signed for Ollie Horgan at Finn Harps in March. His league debut for Harps came on 11 April 2014 in a home draw with Wexford Youths and he went on to make 16 league appearances that season. Caomhin originally re-signed for Harps for the 2015 season but changed his mind and moved to Cliftonville in the Dankse Premier League in Northern Ireland.